The long term objectives of the proposed research program are detailed understanding of the ultrasonic propagation properties of biological systems. Measurements are made of absorption, attenuation, and velocity of ultrasonic waves in tissues, both in vivo and in vitro, and in model systems. Continued development of appropriate instrumentation for obtaining the necessary experimental data is also pursued. The attainment of these objectives contribute to the elucidation of the physical mechanisms at all structural levels of interaction of high frequency sound in biolgical systems, considered essential for the full potential of the ultransonic techniques to be realized for medical diagnostic and therapeutic applications and for biochemical and biophysical investigations of fast reactions. The results of these investigations contribute information immediately applicable to questions regarding toxic effects, dosimetry of clinical ultrasound, efficacy of therapeutic applications, and risk assessment of diagnostic applications. Particular attention will be devoted to studying properties of fetal bone, ovary, lung, and aqueous suspensions of liposomes (as models of biological cells in suspension), during the proposed grant period.